A Systems Approach for the Management of Russian Knapweed (Centaurea repens)

Studies were initiated in Wyoming to determine the potential of grass competition as an alternative to repetitive herbicide treatment or other cultural practices for control of Russian knapweed. An experiment was established to evaluate the effects of five grass species, including Russian wildrye cv. ‘Bozoisky.’ Picloram, applied to Russian knapweed during the first frost, reduced Russian knapweed from an average of 80.1% live canopy cover which equates to 0% control. Untreated, unseeded checks resulted in 83.9 and 81.1% control in tilled and nontilled treated plots, respectively. Grass cover increased in untreated seeded plots from an average of 11.3 and 8.2% in tilled and nontilled plots, respectively, to 65% in tilled and 66% in nontilled plots treated with clopyralid plus 2,4-D. Grass cover also increased 69.7% in tilled and 66% in nontilled plots treated with picloram. There was no significant difference between grass varieties when compared to percent Russian knapweed cover. Reductions to zero live canopy cover of Russian knapweed were obtained with a single application of picloram. Economic feasibility thresholds were obtained from four out of five varieties, including a significant difference provided by nontilled Russian wildrye treated with picloram.

Evaluation of Rust Fungi as Biological Control Agents of WeedyCentaureain North America

FourCentaureaspecies, diffuse knapweed (C.diffusaLam. # CENDI, spotted knapweed (C.maculosaLam. # CENMA), yellow starthistle (C.solstitatilisL. # CENSO), and the closely related Russian knapweed [Acroptilon repens(L.) DC. syn.Centaurea repensL. # CENRE] are serious introduced weeds which infest extensive areas of pasture and rangeland in Canada and the United States (1, 10, 14, 17, 18, 26, 30). Russian knapweed and yellow starthistle are also troublesome in cultivated land (1, 14, 26). These four species are members of the Cardueae tribe of the Asteraceae family and have been accidentally introduced from Eurasia as contaminants in crop seed.

Distribution of Yellow Starthistle (Centaurea solstitialis) and Russian Knapweed (Centaurea repens)

The world distributions of yellow starthistle (Centaurea solstitialis L. ♯ CENSO) and Russian knapweed (Centaurea repens L. ♯ CENRE) are mapped based on information from world regional floras. A survey of weed specialists and herbaria was made in the United States, and the extent of the infestations and economic importance of these weeds was determined for the 48 contiguous states. Yellow starthistle currently occurs in 209 counties in 23 states and has apparently increased in abundance since 1970. Russian knapweed was reported in 412 counties in 21 states and does not seem to be spreading. Both of these weedy plant species are of greater economic importance in the western United States than in the east.

THE BIOLOGY OF CANADIAN WEEDS.: 43. Acroptilon (Centaurea) repens (L.) DC.

Russian knapweed (Acroptilon repens (L.) DC.) is an introduced persistent perennial weed species which is commonly found in cultivated land in western Canada and infrequently in southern Ontario. The weed is characterized by its extensive root system, relatively low seed production, and persistence. Cultural, chemical and biological control strategies of the species are reviewed. This summary of the biological information of Russian knapweed is presented as part of a series on the biology of Canadian weeds.

Autoecious Puccinia species attacking Cardueae in North America

Native rusts include P. irrequiseta on Centaurea americana and the following taxa on Cirsium: P. laschii var. laschii and var. xerophila var. nov.; P. altissimorum sp. nov. and var. horriduli var. nov.; P. californica and var. deserticola var. nov.; P. inclusa and vars. flodmanii, boreohesperia, brevifolii, and mexicana vars. nov. Introduced rusts include P. bardani on Arctium minus, P. carthami on Carthamus tinctorius, P. acroptili on Acroptilon (Centaurea) repens, P. centaureae var. centaureae on Centaurea nigra, P. cyani var. cyani and var. sublevis var. nov. on Centaurea cyanus, P. cnici var. cnici on Cirsium vulgare, and P. punctiformis on Cirsium arvense. The origins and relationships of the American Cirsium rusts are discussed in relation to those of the host plants. A hitherto disregarded adaptation to arid climates, the evolution of very fine and close urediniospore echinulations, is described and exemplified; it is suspected to function through improved retention of spores in the sorus in dry weather when susceptible young leaves are unavailable. Biological observations suggest means of limiting the spread of rusts of economic hosts and promoting those of some weedy hosts.

A GROWTH INHIBITOR FOUND IN CENTAUREA SPP.

The growth of tomato and barley plants was inhibited in soils naturally infested with knapweed or in soils artificially infested with powdered knapweed residues. The leaves contained a higher proportion of the toxic material than did other plant parts, and the leaves of Centaurea repens L. were more inhibitory than those of Centaurea diffusa Lam. and Centaurea maculosa Lam. Chromatographic techniques were developed to isolate an inhibitor common to the three species which was soluble in both water and ether, and inhibitory to the seedling development of barley and lettuce. The material was relatively more inhibitive to root development than to the top growth of the species tested. Chromogenic sprays and ultraviolet absorption spectra indicated that the inhibitor was an indole derivative. The presence of a plant growth inhibitor in these Centaureas may partially explain their rapid establishment in almost pure stands.